A delay line deflection structure is a deflection structure of the traveling wave type used in electron discharge tubes, such as cathode-ray tubes for high frequency oscilloscopes, to reduce the magnitude of deflection signal velocity in the propagation direction of electrons in the electron beam. Traveling wave delay line deflection structures generally comprise a pair of deflection members disposed on opposite sides and extending along the path of an electron beam. An electric field varying in intensity and direction in accordance with the magnitude and polarity of the deflection signal deflects the electron beam. A delay is introduced to reduce the speed of deflection signal propagation along the deflection structure until such speed equals that of the beam electrons, thereby allowing accurate beam deflection by high frequency signals.
Delay line deflection structures generally include meander line and helical deflection structures. By reason of its design, a helical deflection structure has an inherent capability of providing characteristic impedances exceeding those obtainable in meander line deflection structures. However, in conventional delay line structures, longitudinal voltage gradients of increasing intensities develop along the length of the deflection structure in proportion to an increase in the frequency of the deflection signal for frequencies of greater than 1 GHz. Since the potentials applied to the pair of deflection members are of opposite polarity, the voltage gradients in the direction of electron beam travel are positive for one of the deflection members and negative for the other deflection member. Consequently, electrons on the side of the electron beam adjacent to one of the deflection members are accelerated while electrons on the side of the beam adjacent to the other deflection member are de-accelerated. The result of these acceleration and de-acceleration effects is a significant defocusing of the electron beam and a deterioration of display resolution.
It is, therefore, an object of the present invention to provide in an electron discharge tube such as a cathode-ray tube a deflection structure that substantially eliminates the phenomenon of defocusing that results from longitudinal voltage gradients at very high frequencies.
It is another object of the present invention to provide such a deflection structure that produces accurate deflection effects, is reliable in service, and is economically feasible to manufacture.